This invention relates to firing systems for use with pulverized solid fuel-fired furnaces, and more specifically, to a minimum recirculation flame control (MRFC) solid fuel nozzle tip for use in such firing systems.
It has long been known in the prior art to employ pulverized solid fuel nozzle tips in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces. By way of exemplification and not limitation in this regard, reference may be had to U.S. Pat. No. 2,895,435 entitled "Tilting Nozzle For Fuel Burner", which issued on Jul. 21, 1959 and which was assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 2,895,435, there is provided a tilting nozzle that is alleged to provide substantially uniform distribution of the secondary air mixture leaving the tilting nozzle and substantially uniform velocity across the discharge opening of the tilting nozzle into the furnace. To this end, the tilting nozzle includes an inner conduit 5 within an outer conduit 6. Moreover, a plurality of baffles or division walls 17, 18 and 19 are provided within the inner conduit 5 arranged in planes substantially parallel to fluid flow and such as to divide the inner conduit 5 into a multiplicity of parallel channels. These baffles or division walls 17, 18 and 19 are designed to be operative to correct the concentration of the air-fuel mixture along the deflecting wall of the inner conduit 5 and the resulting relatively unequal pressure there when the titling nozzle is tilted. Thus, the effect is that as the tilting nozzle is tilted, either upwardly or downwardly, the unequal velocities through the tilting nozzle are made substantially equal by restricting the flow in the high pressure zone present at the inlet end of the inner conduit 5 and encouraging the flow in the low pressure zone also present at the inlet end of the inner conduit 5.
Another prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 4,274,343 entitled "Low Load Coal Nozzle", which issued on Jun. 23, 1981 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,274,343, there is provided a fuel-fired admission assembly of the type incorporating a split coal bucket having an upper and a lower coal nozzle pivotally mounted to the coal delivery pipe and independently tiltable of each other. Continuing, a plate is disposed along the longitudinal axis of the coal delivery pipe with its leading edge oriented across the inlet end of the coal delivery pipe so that that portion of the primary air pulverized coal stream having a high coal concentration enters the coal delivery pipe on one side of the plate and that portion of the primary air-pulverized coal stream having a low coal concentration enters the coal delivery pipe on one side of the plate and that portion of the primary air-pulverized coal stream having a low coal concentration enters the coal delivery pipe on the other side of the plate. Moreover, the trailing edge of the plate is orientated across the outlet end of the coal delivery pipe such that that portion of the primary air-pulverized coal stream having a high coal concentration is discharged from the coal delivery pipe through the upper coal nozzle and such that that portion of the primary air-pulverized coal stream having a low coal concentration is discharged from the coal delivery pipe through the lower coal nozzle.
Still another prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 4,356,975 entitled "Nozzle Tip For Pulverized Coal Burner", which issued on Nov. 2, 1982 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,356,975, there is provided a nozzle tip having one or more splitter plates disposed therein, which is characterized in that the splitter plates comprise a first plate of highly abrasion resistant material disposed at the inlet end of the nozzle tip and a second plate of highly heat resistant material disposed at the outlet end of the nozzle tip. Furthermore, the first plate of highly abrasion resistant material has its leading edge, which is preferably rounded, disposed along the inlet end of the nozzle tip and extends a substantial distance through the inner shell of the nozzle tip along a line parallel to the longitudinal axis thereof. Also, the highly abrasion resistant plate terminates within the nozzle tip with its trailing edge set back from the discharge end of the nozzle tip. Moreover, the second plate of highly heat resistant material is disposed within the inner shell so as to abut the trailing edge of the highly abrasion resistant plate and extends therefrom towards the discharge end of the nozzle tip along a line parallel to the longitudinal axis thereof.
Still a further prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is to be found depicted in U.S. Pat. No. 4,434,727 entitled "Method For Low Load Operation Of A Coal-Fired Furnace", which issued on Mar. 6, 1984 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,434,727, there is provided a fuel-air admission assembly whereby the primary air and pulverized coal mixture discharging into the furnace is split into two independent coal-air streams when the furnace is operated at low loads such as during the minimum demand periods. Furthermore, the split primary air and pulverized coal streams are independently directed into the furnace in angular relationship away from each other. Thus, in doing so an ignition stabilizing pocket is established in the locally low pressure zone created between the spread apart coal-air streams. Accordingly, hot combustion products are drawn, i.e., recirculated, into this low pressure zone, thereby providing enough additional ignition energy to the incoming fuel to stabilize the flame.
Yet another prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 4,520,739 entitled "Nozzle Tip For Pulverized Coal Burner", which issued on Jun. 4, 1985 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,520,739, there is provided a nozzle tip for a burner on a pulverized coal-fired furnace for receiving a stream of pulverized coal and air discharging from the coal delivery pipe of the burner and directing the pulverized fuel and air stream into the furnace. This nozzle tip is comprised of a base body, a replaceable highly abrasion resistant insert, and a replaceable highly temperature resistant end cap that is readily attachable by mechanical means to the base body with the abrasion resistant insert disposed therein. Moreover, the insert defines a highly abrasion resistant flow conduit through the nozzle tip from the discharge end of the base body to the receiving end of the end cap through which the pulverized fuel and air stream passes from the burner into the furnace.
Yet still another prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 4,634,054 entitled "Split Nozzle Tip For Pulverized Coal Burner", which issued on Jan. 6, 1987 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,634,054, there is provided a nozzle tip for a burner on a pulverized fuel-fired furnace that is alleged to be particularly adapted to provide improved ignition stability during low load operation of the furnace. This nozzle tip comprises an open-ended inner shell defining a flow passageway through which a mixture of pulverized fuel and transport air passes from the burner into the furnace, an open-ended outer shell spaced from and surrounding the inner shell thereby defining an annular flow passage therebetween through which additional air for combustion passes from the burner into the furnace and plate means disposed within the inner shell for dividing the flow passageway therethrough into first and second flow passages that extend from the inlet of the inner shell to the outlet of the inner shell in a diverging manner with a void region established therebetween through which flow is precluded. By virtue of the construction thereof, the coal-air mixture discharging from the burner is split by the plate means into a first stream that is directed into the furnace through the first flow passageway through the inner shell and a second stream that is directed into the furnace through the second flow passageway of the inner shell. Thus, the coal-air mixture is directed into the furnace in two diverging streams. As such, in doing so an ignition stabilizing pocket is established in the locally low pressure zone created between the spread-apart and diverging coal-air streams in the furnace just downstream of the void region established between the diverging first and second flow passageways through the inner shell of the nozzle tip. Accordingly, coal is concentrated in this pocket and hot combustion products are drawn back into the pocket from the flame to provide additional ignition energy to the incoming fuel to stabilize the flame.
Yet a further prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 5,315,939 entitled "Integrated Low NO.sub.X Tangential Firing System", which issued on May 31, 1994 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,315,939, there is provided a fuel nozzle that embodies a flame attachment pulverized solid fuel nozzle tip. The principal function of this flame attachment pulverized solid fuel nozzle tip is stated to be that of effecting the ignition of the pulverized solid fuel being injected therefrom into the burner region of the pulverized solid fuel-fired furnace at a point in closer proximity, i.e., within two feet thereof, than that at which it has been possible to effect ignition heretofore with prior art forms of pulverized solid fuel nozzle tips. Moreover, this flame attachment pulverized solid fuel nozzle tip is characterized principally by the bluff-body lattice structure, which is provided at the discharge end thereof. This lattice structure is said to change the characteristics of the pulverized solid secondary air stream, which is being discharged from the flame attachment pulverized solid fuel nozzle tip, from principally laminar flow to turbulent flow. The increased turbulence in the pulverized solid fuel/air stream increases the dynamic flame propagation speed and combustion intensity. This in turn results in rapid ignition of the entire pulverized solid fuel/air jet (close to the flame attachment pulverized solid fuel nozzle tip but not attached thereto), high early flame temperature (maximize volatile matter release including fuel nitrogen) and rapid consumption of available oxygen (minimize early NO formation). The real benefit and commercial significance of the flame attachment pulverized solid fuel nozzle is stated to reside in its ability to provide excellent performance without having an attached flame. It is further stated that experience has shown that prior art forms of flame attachment nozzle tips can suffer premature failure and/or pluggage problems when firing certain pulverized solid fuels. To this end, since this flame attachment pulverized solid fuel nozzle tip can maintain a stable detached flame, it is said to be capable of obviating the pluggage/rapid burn-up problems, which have served to disadvantageously characterize the prior art forms of flame attachment nozzle tips that have been employed heretofore.
Although the pulverized solid fuel nozzle tips that form the subject matter of the issued U.S. patents to which reference has been had hereinbefore have been demonstrated to be operative for their intended purposes, there has nevertheless been evidenced in the prior art a need for such pulverized solid fuel nozzle tips to be further improved. In this regard, it has been found that pulverized solid fuel deposits, i.e., coal deposits, on and within the pulverized solid fuel, i.e., coal, nozzle tips are problematic from an operational standpoint. That is, such coal deposits on and within the coal nozzle tip have been found to lead to either premature or catastrophic coal nozzle tip failure, depending primarily upon the tenacity of the formed deposits and the rate at which the deposition occurs. To this end, deposition of coal on or within the coal nozzle tip is believed to be caused by a combination of the following three variables: 1) coal composition/type, i.e., slagging, non-slagging, sulfur/iron content, plasticity, etc.; 2) furnace/coal nozzle operational settings, i.e., primary/secondary air flow rate/velocity, tilt position, firing rate, etc.; and 3) coal nozzle tip aerodynamics.
Thus, by way of summary, present designs, i.e., prior art forms, of coal nozzle tips have by and large been found to exacerbate the coal deposition problem through the creation of regions of low or negative velocities, i.e., recirculation, that cause slowly moving, "hot", coal particles to come in contact with "hot" coal nozzle tip metal surface. Namely, it has been found that as a result of this interaction, and under requisite thermal conditions that are related to the coal's plasticity, some of the coal particulate sticks to the plate, thus initiating the deposition process. Moreover, with specific reference to present designs, i.e., prior art forms, of coal nozzle tips, it has been found that regions of low and negative velocities typically occur along the thickness of the nozzle plane platework and in the sharp corners of the primary air shroud.
There has, therefrom, been evidenced in the prior art a need for a new and improved pulverized solid fuel nozzle tip that would address the deficiencies from which present designs, i.e., prior art forms of pulverized solid fuel nozzle tips have been found to suffer. Namely, there has been evidenced in the prior art a need for a new and improved pulverized solid fuel nozzle tip that would be advantageously characterized in the following respects: 1) would minimize low and negative, i.e., recirculation, velocity regions at the exit plane of the pulverized solid fuel nozzle tip, 2) would reduce available deposition surface on the pulverized solid fuel nozzle tip, and 3) would vary the nozzle tip/solid fuel nozzle thermal conditions to keep the "hot" solid fuel particulate matter from deposition on available metal platework surfaces of the pulverized solid fuel nozzle tip. Such a new and improved pulverized solid fuel nozzle tip accordingly would be effective in controlling the deposition phenomena, from which present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips have been found to suffer. This would be accomplished through the aerodynamic design embodied by such a new and improved pulverized solid fuel nozzle tip coupled with proper adjustment of the controllable operational variables, i.e., secondary air flow rate, etc. As employed herein, the term "controllable" refers to the fact that solid fuel type and furnace load, and in some, notably retrofit, cases primary air flow rate are typically not controllable operational variables for mitigation of the deposition phenomena.
To this end, such a new and improved pulverized solid fuel nozzle tip would be advantageously characterized by the fact that certain features were collectively embodied thereby. A first such feature is that the primary air shroud would be recessed. Recessing the primary air platework, i.e., primary air shroud, to within the exit plane of the secondary air shroud would remove this potential deposition surface from the firing zone, i.e., the exit plane of the nozzle tip, and would provide some cooling via the shielding effect of the secondary air shroud. Additionally, a shorter primary air plate, i.e., primary air shroud, would reduce the contact surface for heat transfer thereto and deposition thereon of coal particles. A second such feature is that the splitter plates would be recessed. Recessing the splitter plates along with the primary air shroud to within the exit plane of the secondary air shroud would remove this potential deposition surface from the firing zone, i.e., the exit plane of the nozzle tip, and would provide some cooling via the shielding effect of the secondary air shroud. Additionally, shorter splitter plates would reduce the contact surface for heat transfer thereto and deposition thereon of coal particles. A third such feature is that the secondary air shroud support ribs would be recessed. Recessing the secondary air shroud support ribs would keep the circulation region, and vertical deposition surface normally created by these devices at the exit of the nozzle tip from the firing zone, thus reducing their possible influence in the deposition process. Structurally, recessing the secondary air support ribs would also allow the front portions of the secondary air and primary air shrouds to independently expand reducing thermally induced stress. A fourth such feature is that the trailing edge of the primary air shroud would be tapered. Tapering the trailing edge of the primary air shroud would reduce the recirculation region created by the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Such a recirculation region draws hot particulate matter back to the vertical plate surface creating or exacerbating the coal deposition phenomena. Also, such a recirculation region can provide conditions conducive to combustion, thus creating flames within the recirculation region, which raise temperatures and further exacerbate the deposition problem.
To this end, the primary air shroud platework would be tapered at a small enough angle such that neither the secondary air nor the primary air flows separate from the plate thus obviating the creation of additional, unwanted recirculation. A fifth such feature is that the splitter plate ends would be tapered. The splitter plate ends would be tapered to reduce the recirculation region created by the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips, and the shed vortices created by the blunt faced leading edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. As in the case of the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips, the recirculation region induced by the blunt faced splitter plate of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips draws hot particulate back to the vertical plate surface creating or exacerbating the coal deposition phenomena. Also, such a recirculation region can provide conditions conducive to combustion, thus creating flames within the recirculation region, which raise temperatures and further exacerbate the deposition problem. In addition, the vortices induced by the blunt faced leading edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips increase turbulence levels within the primary stream thus exacerbating coal particulate deposition. To this end, the splitter plate edges would be tapered at a small enough angle to avoid primary air separation, which would create additional, unwanted flow recirculation. A sixth such feature is that the secondary air shroud would embody a bulbous inlet. The bulbous inlet of the secondary air shroud would minimize secondary air bypass of the fuel air shroud during tilt conditions which currently occurs with present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Moreover, the bulbous inlet would enhance secondary air flow through the fuel air shroud thereby acting to both cool the nozzle tip platework, and thermally blanket the primary air/coal stream to delay ignition, which also provides a tip cooling effect. On the other hand, were the secondary air shroud flow to be allowed to drop severely due to tip bypass, low pressure/velocity regions could be created within the secondary air shroud, leading to reverse flow and particle deposition within this annular region. A seventh such feature is that the primary air shroud exit plane corners would be rounded. Rounding the primary air shroud exit plane corners increases the corner velocities with respect to that found in the ninety degree corners of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Increasing the corner velocities increases the erosion energy for air/coal flowing through this region to help remove active deposits, and otherwise avoid deposition. Also, the rounded corners decrease the available surface for heat transfer from the hot platework to the cooler air/coal mixture for a volume element of air/coal within the tip corner. An eighth such feature is that the secondary air shroud exit plane corners would be rounded. The rounded secondary air shroud exit plane corners, combined with the rounded primary air shroud exit plane corners, provide for higher corner velocities, thus minimizing low velocity regions on the secondary air shroud. In addition, the rounded secondary air shroud exit plane corners assist in achieving a uniform secondary air opening. A ninth such feature is that a uniform secondary air shroud opening (exit plane) would be provided. Providing a uniform secondary air shroud opening provides for uniform secondary air distribution within the nozzle tip. Namely, providing a uniform secondary air shroud opening provides for uniform nozzle tip cooling via the secondary air stream, but also provides for uniform blanketing of the primary air stream for control of ignition position and of NO.sub.X emissions. A tenth such feature is that for certain applications wherein minimum NO.sub.X emissions and/or minimum carbon in the flyash are criteria that need to be met, it would be possible to provide a version of such a new and improved pulverized solid fuel nozzle tip embodying collectively all of the nine features that have been enumerated hereinabove, which would enable minimum NO.sub.X emissions and/or minimum carbon in the flyash to be realized, while yet thereby enabling there to be realized concomitantly therewith minimum fuel deposition and therethrough avoidance of pulverized solid fuel nozzle tip failure occasioned thereby. Moreover, such minimization of NO.sub.X emissions and/or minimization of carbon in the flyash would be attainable by providing a version of such a new and improved pulverized solid fuel nozzle tip wherein one or more bluff bodies, each embodying a predefined geometry, are suitably supported in mounted relation at a predetermined location therewithin.
It is, therefore, an object of the present invention to provide a new and improved solid fuel nozzle tip for use in a firing system of the type utilized in pulverized solid fuel-fired furnaces.
It is a further object of the present invention to provide such a new and improved solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is operative as a minimum recirculation flame control (MRFC) solid fuel nozzle tip.
It is another object of the present invention to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the primary air shroud thereof is recessed.
It is still another object of the present invention to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the splitter plates thereof are recessed.
Another object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the secondary air shroud support ribs thereof are recessed.
A still another object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the trailing edge of the primary air shroud thereof is tapered.
A further object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the ends of the splitter plates thereof are tapered.
A still further object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the secondary air shroud thereof embodies a bulbous inlet.
Yet an object of the present invention is to provide such a new and improved MRFC solid nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the exit plane corners of the primary air shroud thereof are rounded.
Yet a further object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the exit plane corners of the secondary air shroud thereof are rounded.
Yet another object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the secondary air shroud thereof is provided with a uniform opening.
Yet still another object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that for purposes of attaining therewith minimum NO.sub.X emissions and/or minimum carbon in the flyash one or more bluff bodies, each embodying a predefined geometry, are suitably supported in mounted relation at a predetermined location therewithin.